1. Field of the Invention
This invention relates to a method for curing a photocurable composition with high covering power, and also to an apparatus for practicing the method.
2. Description of the Prior Art
As for the method for curing photo-curable compositions with high covering power by exposure to ultraviolet ray energy, a Japanese document (Patent Laid Open No. 54135/1973) teaches that such a curing is possible if the intensity of ultraviolet rays is selected suitably and the type of the composition is restricted as specified as to the number of double bonds and molecular weight. However, in the case where the composition contains a large quantity of pigments with high opacity or a coated film thereof is extraordinarily thick, there is a tendency of the composition in failing to be wholly cured, or causing wrinkles on the surface. The reason for this is inferred as follows: The penetration of ultraviolet rays into the coating film is inhibited by the opaque pigments so that this can lead to differential cure and strain within the coating film, in particular between the surface portion and the deeper portion. Then, it causes either crinkles or wrinkles on the surface. For eliminating this phenomenon, the aforementioned Japanese document further teaches that the integral curing of the film of the composition can be attained by increasing the intensity of the ultraviolet rays to such an extent that the rays can reach the deepest part of the film of the composition. However, such a procedure tends to exaggerate the differential cure between the surface and deeper portions, thus causing irregularities on the surface of the film of the composition. It is, also, known that a combination of a fluorescent lamp for photo-curing use and a high-pressure mercury lamp may be employed in curing a photo-curable coating composition having a good ultraviolet ray transmittance, but this method is not suitable for curing a photo-curable coating composition containing a large quantity of pigments with high opacity. That is, two different kinds of light source have nearly the same dominant wavelengths, and a difference in the intensity of the two radiation is utilized. The irradiation intensity of the fluorescent lamp for photo-curing use is so weak in penetrating the photo-curable composition with high covering power that a differential cure between the surface and the deeper portion ofthe coating film is caused and then coating film have the appearance of crinkles or wrinkles as mentioned above results.